Chances are the rep was either wrong, or didn’t explain themselves very well, FTTC VDSL is capped at 40,55 or 80Mb, if your individual line is capable of delivering the highest band, and you have purchased that band, your router will sync at that rate, obviously if your line cannot achieve the upper bands , it’s probably not a good choice to purchase them ( some may do that for the improved upload speed though )
ISP’s cannot market products as they once did in the past ( so no longer should you see ‘upto 80Mb’ ) they have to use ‘average’ speeds a typical customer of theirs on that band get , so in simplest terms , say they had 2 customers both on the highest band , one getting 80Mb, and one getting 60Mb the advert would have to be ‘average speed 70Mb, one getting better than average , one getting less, obviously in reality it’s millions of customers but the process is the same, it could be the rep has quoted what a typical BT customer on Superfast 2 ( or whatever that band is marketed as ) gets, for al, the use it is, personally I think the old ‘upto’ way of advertising was no worse than the average speed they now have to use.
As others have already stated if your line once synced at 80Mb but now syncs at less, it may well be due to cross talk from adjacent circuits , the more people that take the service the more likely cross talk becomes an issue , and DLM may reduce your sync speed to give a stable service, or their could be an issue ( fault) with the ‘line’ , and DLM again has intervened.
For what is worth, I’m on 80Mb, ( around 80 metres from the cab ) and sync at 80Mb, and have done since the service was provided.
